Most airports in Canada don't have a real separation in terms of international and domestic gate areas. Passengers currently transferring international-international at YVR, YYZ etc. still have to get inspected by CBSA (or go through a simplified process if flying certain carriers during certain times of the day). At these airports, it is very easy to walk from a domestic gate to an international gate.

Having used Terminal 5 and Terminal 1 at London Heathrow, I've noticed that they mix both international and domestic departures into the same area of the terminal. This way, both international and domestic passengers can use the same lounges and shopping areas of the terminal. (I had the pleasure of using BA's Concorde Room after arriving from an international BA flight at LHR while connecting to a UK domestic flight!)

To prevent international passengers (especially those in transit to another international flight) from boarding domestic flights, passengers who are departing from Heathrow Terminal 1 and 5 have to get their photo taken. This photo is then matched with them when they board a domestic flight.

Passengers transferring international to domestic also have to get their photo taken after clearing immigration (at a special counter for transit passengers) so that they can be matched when boarding their onward domestic flight.

As for passengers flying to international destinations or transferring from one international flight to another, they can bypass immigration and head directly to the departure gate. Should they attempt to board a domestic flight or leave the secure area they would be denied as they would not have a photo on record.

Would this work for Canada? I certainly think so. By implementing this photo matching process, most Canadian airports can implement sterile international-international transfers without major modifications to current terminal infrastructure.